Apparatus and method for web cooling in a vacuum coating chamber

ABSTRACT

A chill drum ( 14 ) is modified to improve heat transfert between the drum and a flexible web substrate ( 20 ) disposed around the drum. The drum surface ( 22 ) contains a series of passages ( 44 ) and distribution holes ( 46 ). A working gas is injected into these passages and flows out of the distribution holes into the space between the web and drum. A cover ( 32 ) prevents working gas from escaping from frum passages in the area not covered by the web, and supplies the working gas to the passages at the drum cover. Once gas is in the passages, leakage only occurs from the edges of the web. The pressure in the passages remains essentially constant around the drum, producing uniform elevated pressures under the entire web. Elevated pressure behind the web significantly improves overall heat transfert, thereby allowing higher deposition rates and other process advantages.

BACKGROUND

This invention relates to a device and method for improving heattransfer between a web and a chill drum in a vacuum chamber.

Many vacuum deposition processes involving flexible web substrates areaccomplished with the web disposed around a rotating chilled drum. Inthese systems, deposition sources are arrayed around the drum andcontinuously deposit coatings onto the moving web. A limiting parameterof these deposition processes is the heat imparted to the web during thecoating process. If the heat applied by the process exceeds maximum webparameters, the web wrinkles or is otherwise damaged. Many productstoday are either expensive or are not produced because of low depositionrates dictated by insufficient web heat transfer.

Heat removal from a web to a chilled drum is primarily limited by theinterface between the web and drum. In this interface, heat istransferred by three modes. One mode is conduction between the twosurfaces. Typical polymer webs are not smooth at the micron level. Theyare made intentionally rough to allow the film to be wound on a spool.While improving ease of handling, this surface roughness greatly reducesthe actual contact between the web and the drum. Lack of contact in turnlimits the heat transfer by conduction to less than 5% of the total heattransfer. A second heat transfer mode is radiation. While alsocontributing to heat removal, heat removed by this mode is limited bythe relatively small temperature difference possible between the web anddrum.

The third and largest contributor to heat transfer between the web anddrum is molecular conduction. This mode occurs when molecules trappedbetween the web and drum transfer heat between the two surfaces.Commonly water vapor is present in polymer substrate films and devolvesfrom the substrate during the deposition process. A portion of thiswater vapor is trapped between the web and drum and provides a mediumfor molecular conduction heat transfer. Important factors determiningthe rate of molecular conduction heat transfer include: the temperatureof the web and drum, the web and drum materials, the type of gas, andthe pressure of the gas. Variations to web and drum temperatures andmaterials are limited by materials properties; however, a significantopportunity for improvements in heat transfer resides with the type ofgas and the pressure of the gas. If these variables can be optimized,the heat load into the web can be increased without damage to the web.Because the pressure can be varied by orders of magnitude, it offers thebest lever for dramatic heat transfer improvement.

Several prior-art devices have attempted to improve the web-to-drum heattransfer by elevating the pressure between the web and drum:

U.S. Pat. No. 3,414,048 (Rall) discloses a drum with built in normallyclosed valves. Web in contact with the drum forces open the valves,allowing gas to flow into the gap between the web and drum. Thisapparatus is complicated with many parts to stick or fail. Also a thinpolymer web may fail to exert sufficient pressure on the valve to openthem. Other limitations of this approach include hot spots on the web(the valves are not cooled) and non-uniformity of web cooling.

U.S. Pat. No. 5,076,203 (Vaidya) discloses apparatus to increase thepressure behind the web by blowing gas into the gap with a nozzlearrangement. Another method to increase pressure employs a porous metalnon-rotating section through which gas is distributed. An enclosurearound the web and drum at the entrance point of the web is shown as ameans to limit the increase in chamber pressure as gas is urged into thegap. While informative, several faults limit the utility of this deviceand method:

-   -   No means is described to continually trap working gas behind the        web in a rotating drum configuration. Due to the minute quantity        of gas trapped as the drum rotates away from the nozzle area,        most or all trapped gas is lost before reaching the rewind side        of the chamber. Therefore, the enclosure that is used to prevent        gas from raising the system pressure only envelopes the unwind        drum side. This indicates the small amount of gas actually urged        into the web. High pressures would result in large gas loads on        the chamber vacuum system.    -   No means is disclosed to bring a working gas into a porous        material where the porous material is applied to the surface of        a rotating drum.    -   No means is disclosed to alter the pressure effectively across        the width of the web for the purpose of controlling web heat        transfer and conveyance parameters. Distribution of gas in the        porous surfaces is across the width of the drum.

In U.S. Pat. No. 5,395,647 (Krug), a vapor such as water is condensedonto the web prior to contact with the chill drum. While recognizing theneed to improve heat transfer between the web and drum, this methodlacks practicality for most deposition processes. The use of liquidwater creates an undesirably large gas load on the pumping system, anduniformly dispensing of water vapor in vacuum is difficult. While cleanand of a suitable vapor pressure, water vapor is detrimental to theformation of many desirable films. If a low vapor pressure fluid otherthan water is used, the web becomes contaminated with the substance.

SUMMARY

By way of general introduction, the web coating apparatus describedbelow includes a rotatable drum that carries a web past a coatingdeposition station in a vacuum chamber. Tension on the web presses itagainst the drum over a first arc that includes the coating region ofthe apparatus, and the web is spaced from the drum in a second arcdisposed opposite the coating deposition station. The drum is chilled,as for example with conventional liquid cooling, and the drum isprovided with an array of passages. These passages open out onto the websupport surface via exit portions that are continuously open. A sourceof pressurized gas is coupled with these passages such that pressurizedgas is pumped via the passages and the exit portions into the regionbetween the web and the drum. This pressurized gas improves heattransfer between the web and the drum.

In order to reduce undesired leakage of gas out of the passages over thearc of the drum not covered by the web, a set of seals is provided.These seals seal the exit portions of the passages in a sealing regiondisposed opposite the coating deposition station. These seals can slideover the working surface of the drum (or alternately they can sealwithout contacting the working surface of the drum or the web that issupported by the working surface of the drum), thereby preventingpressurized gas in the passageways from escaping out of the drum intothe region of the vacuum chamber adjacent the coating depositionstation.

The foregoing paragraphs have been provided by way of generalintroduction, and they are not intended to narrow the scope of thefollowing claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic plan view of a web coating apparatusthat incorporates a first preferred embodiment of this invention.

FIG. 2 is an enlarged view of selected portions of the apparatus of FIG.1.

FIG. 3 is a perspective view of the drum and drum cover of the apparatusof FIGS. 1 and 2.

FIG. 4 is a cross-sectional view taken along line 4—4 of FIG. 2.

FIG. 5 is an exploded perspective view of an alternative drum suitablefor use in the apparatus of FIG. 1.

FIG. 6 is a fragmentary perspective view of another alternative drumsuitable for use with this invention.

FIG. 7 is a fragmentary cross-sectional view taken along line 7—7 ofFIG. 6.

FIG. 8 is a fragmentary plan view of a portion of the web supportsurface of another preferred embodiment.

FIG. 9 is a cross-sectional view taken along line 9—9 of FIG. 8.

FIG. 10 is a fragmentary plan view of a portion of the web supportsurface of yet another alternative embodiment of this invention.

FIG. 11 is a cross-sectional view taken along line 11—11 of FIG. 10.

FIG. 12 is a schematic plan view of another preferred embodiment of thisinvention, in which seals seal against the web rather than the websupport surface of the drum.

FIG. 13 is a fragmentary schematic sectional view of another embodimentthat provides gas at different pressures at different axially spacedregions of the web support surface.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

General Discussion

The present invention relates to an improved apparatus and method forincreasing heat transfer between a flexible web and a chill drum in avacuum chamber. This invention is particularly useful in improving heattransfer in systems employing sources to deposit coatings onto the web.

In one embodiment, a liquid-cooled drum is constructed with a seriespassages around the drum perimeter with connecting tubes to the surfaceof the web. These passages and tubes are proximal to the surface and areseparate from the liquid cooling. A working gas is constantly introducedinto the passages, raising the pressure inside the passages. Thepassages are of a sufficient size such that the conductance of thepassages far exceeds the leakage conductance from the edges of the web.For this reason, the passage pressure remains essentially constantthrough the entire length of the passage around the drum. Connectingtubes between the passages and drum surface allow gas to flow into thegap between the web and drum. The conductance limitation is the leakagefrom the edges of the web, and the pressure under the web thereforeequals passage pressure. For example, pressures of 10 Torr or greatercan be maintained between the web and drum while deposition zonepressures remain in the 5×10−4 Torr range. This is achieved because onlya minimal gas load is added to the deposition zone vacuum pumpingrequirements. This embodiment has several significant benefits, and itmakes feasible previously-unachievable deposition rates and processes.These benefits include the following:

-   -   The working gas is distributed to produce a uniform pressure        around the entire drum perimeter rather than only at one        deposition zone. Effective heat removal is possible with systems        employing multiple deposition sources installed around the drum        (e.g. multiple sputter cathodes).    -   Pressure behind the web can optionally be controlled across the        width of the web to optimize heat transfer, pumping loads and        web effects such as wrinkling.    -   With uniform gas distribution and pressure behind the web, local        web tension variations are mitigated. In typical chill drum        deposition systems, hot spots on the web cause local web        expansion and loss of optimum thermal contact. Web wrinkling and        other permanent damage can result. The constant pressure        delivered by the gas distribution network described below        reduces hot spot occurrences.    -   The pumping load on the vacuum system is minimized. While        considerable pressure can be maintained in the gap between the        web and drum, only a small gas load is added to the vacuum        system. For example, With a typical ˜2 um gap between the web        and drum due to web and/or drum roughness, a 10 Torr passage        pressure leaks at only 0.002 Torr-liter/sec into the chamber.    -   The modifications required to a drum and web coater do not rule        out retrofit to an existing coater. Extensive pumping and        winding changes are not required. A standard construction chill        drum can be modified with a network of passages and holes. The        sealing and gas inlet cover can be retrofitted into an existing        system, and the increased gas load on the vacuum pumps is        minimal.    -   Rather than accept water vapor already in the web as the only        heat transfer agent, a choice of gases can be used. In some        cases a process gas can be used, e.g. the gas for sputter        deposition. In this example the leakage from the web edges        becomes part of the process gas delivery system.

Specific Implementations

Turning now to the drawings, FIG. 1 shows a schematic plan view of a webcoating apparatus 10 that includes a vacuum chamber 12. Mounted insidethe vacuum chamber 12 are a rotatable drum 14 and winding hubs 16, 18. Aweb 20 to be coated is initially wound on the winding hub 16 and iswrapped partially around the drum 14, supported by a web support surface22 of the drum 14. The rotating drum 14 transports the web 20 from thewinding hub 16 past a coating deposition station 24 to the winding hub18. The coating deposition station 24 can include one or more sources26. In this example three sputter cathode magnetron sources are used,though more or fewer sources can be used of any suitable type. Theregion of the vacuum chamber 12 in the vicinity of the coatingdeposition station 24 will be referred to in the following descriptionas a coating region 28.

The web coating apparatus 10 also includes a sealed region 30 thatextends over an arc of the drum 14 that is angularly spaced from thecoating region 28 and the coating deposition station 24. In thisexample, the sealed region 30 is bounded by a drum cover 32 thatincludes sliding seals 34 that slide against the web support surface 22immediately adjacent the two lines of contact between the web 20 and theweb support surface 22 as the web 20 approaches the drum 14 on one sideof the drum cover 32 and moves away from the drum 14 on the other sideof the drum cover 32.

In this example, a source 36 of pressurized gas is in fluidcommunication with the sealed region 30 bounded by the drum cover 32 andthe web support surface 22 of the drum 14. A pressure gauge 40 allowsthe pressure of gas in the region 30 to be monitored.

FIG. 2 provides an enlarged view that shows the relationship between thedrum cover 32, the sliding seals 34 and the drum 14. As shown in FIG. 2,the drum 14 includes liquid cooling passages 42 through which a coolingliquid is pumped by conventional means (not shown). In this example, thedrum 14 also includes an array of passages 44. Each passage 44 extendscircumferentially completely around the drum 14, and each of thepassages 44 is in fluid communication with the region immediatelyradially outwardly spaced from the web support surface 22 via exitportions 46. In this embodiment, the exit portions 46 are each shaped asa respective connecting tube that is constantly open between thecircumferential passage 44 and the exterior of the drum 14. With thisarrangement, the drum cover 32 and the sliding seals 34 seal the exitportions 46 in the region of the web support surface 22 not covered bythe web 20. The web 20 itself seals the exit portions 46 over the arc ofthe web support surface 22 covered by the web 20.

In this embodiment, the pressurized gas from the source 36 passes intothe region 30 bounded by the drum cover 32 and creates a workingpressure P1. The pressure P1 is greater than the pressure P2 in thepassages 44, such that the pressurized gas continuously flows from thesource 36 through the region 30 and the exit portions 46 into thepassages 44. The drum cover 32 cooperates with the web 20 to contain thepressurized gas at an elevated pressure inside the passages 44. Workinggas is continually flowing into these passages via the region 30,thereby making up leakage loss and maintaining a substantially constantpressure P2 in all of the passages 44. In this way, gas pressure in thepassages 44 is maintained at a substantially uniform level at all pointsof contact between the web 22 and the drum 14.

FIG. 3 shows a perspective view of the drum 14 and the drum cover 32.The exit portions 46 in the web support surface 22 are illustrated, asis one of the passages 44. Though not shown, a similar passage 44 ispositioned under each of the exit portions 46. In this example, thepassages 44 are circumferentially oriented and extend around the entireperimeter of the drum 14, and the exit portions 46 are shaped as smalltubes. As shown in FIG. 3, the sliding seals 34 at the edges of the drumcover 32 are in direct sliding contact with the web support surface 22of the drum 14. The spacing for the passages 44 and the exit portions 46is determined by the heat transfer requirements of the particularapplication. In this embodiment, the drum cover 32 acts as a manifoldcoupling the source of pressurized gas with the exit portions 46positioned in the sealed region 30 under the drum cover 32. In oneexample, the drum cover 30 and the associated manifold extend over theentire working width of the drum 14.

FIG. 4 provides an enlarged sectional view of two of the passages 44 andtwo of the exit portions 46. The passages 44 ring the web supportsurface 22 and are of a depth and width such that the conductance lossthrough each passage 44 is small compared to the loss through the gapbetween the web 20 and the drum 14. In this way, the pressure in each ofthe passages 44 is maintained at a substantially constant level alongthe entire length of the passage 44 around the drum 14.

The exit portions 46 are small in comparison to the passages 44, and theexit portions 46 connect the passages 44 to the web support surface 22.The exit portions 46 are sized to present a sufficient conductance forgas to flow from the sealed region into the passages 44 while minimizingthe effects on the web 20 caused by interruptions in the web supportsurface 22. The spacing of the passages 44 across the width of the drum14 and the spacing of the exit portions 46 around the perimeter of thedrum 14 are selected to create the desired pressure distribution underthe web 20. In one embodiment, the passages 44 and exit portions 46 arespaced to provide substantially constant pressure in the entire regionbetween the web 20 and the drum 14. In another embodiment, the passages44 and the exit portions 46 are spaced more closely in one portion ofthe web support surface 22 than another, axially-spaced portion of theweb support surface 22. This design produces elevated pressures ofselected gasses in a selected pressure distribution pattern under theweb 20.

Of course, many variations are possible to the preferred embodimentdescribed above. As shown in FIG. 5, the drum 14 can be provided withpassages 44′ that extend axially along the length of the drum 14 ratherthan circumferentially as described above. In this case pressurized gasis introduced into the passages 44′ via a manifold 50 at one end of thedrum 14. In this case, the drum cover 32 does not require connection tothe source of pressurized gas.

As shown in FIG. 6, the exit portions 46′ may be shaped as elongatedslits rather than the tubes described above. In general, the passages44, 44′ can be oriented at any desired angle rather than simply at thecircumferential and axial angles illustrated and discussed above, andthe exit portions 46, 46′ can be shaped as any desired combination oftubes, slits, and other elongated shapes extending partially or fullyalong the length of the associated passages.

FIGS. 8 and 9 relate to one method for forming the passages 44. In thisexample the drum 14 is first provided with a groove 52, and this groove52 is then sealed with a strip 54 that includes indentations 56. Thesestrips 54 and indentations 56 cooperate with the adjacent drum 14 toform the passages 44 and the exit portions 46.

Another alternative arrangement is shown in FIGS. 10 and 11, in whichthe drum 14 is provided with a groove 56 that is closed with weldmaterial 58. This weld material 58 is then drilled to form the exitportions 46.

FIG. 12 relates to another alternative embodiment, in which seals 34′are positioned in close proximity to the web 20, which is in turn ispressed against the web support surface 22. In this embodiment, theseals 34′ are positioned over the web 20 rather than the drum 14. Asbefore, the seals 34′ define a sealed region 30′ that is separated fromthe coating region 28′. To avoid damage to the web surface, the seals34′ do not contact the web but are positioned in close proximity to theweb to limit the gas conductance between sealed region 30′ and coatingregion 28′. In this example, the sealed region 30′ includes the entirewinding zone, which is maintained at a sufficiently high pressure toforce gas via the exposed exit portions into the passages describedabove. Non-contacting seals of the type shown in FIG. 12 may be used inthe embodiment of FIG. 1 instead of the sliding seals 34 to reduceconductance between the drum 14 and the drum cover 32.

FIG. 13 shows another alternative that is closely related to theembodiment of FIGS. 1 and 2. In this case, the cover 32′ includes aseptum 60 that divides the region between the drum cover 32′ and thedrum 14 into two sealed regions 30″ and 30′″. In this case the gassource 26′ supplies pressurized gas to two pressure regulators 62, 64.The pressure regulator 62 is in fluid communication with the sealedregion 30″, and the pressure regulator 64 is in fluid communication withthe sealed region 30′″. The septum 60 forms a seal against the websupport surface 22, and in this way separate pressures P3, P4 aremaintained in the sealed regions 30″, 30′″ respectively. These differentpressures P3, P4 result in a desired pressure distribution along theaxial length of the drum 14 in the respective passages 44. Of course,three, four or more sealed regions and associated pressure regulatorscan be provided to achieve substantially any desired pressuredistribution across the width of the web.

The approach shown in FIG. 13 is useful in applications where heat loadson the web are not uniform across the width of the web or where thereare specific web transport requirements.

In another variation, the spacing of the passages 44, 44′ and the exitportions 46, 46′ may be varied to produce desired temperature patternson the web.

The embodiments described above implement a method for cooling the web20. The drum 14 is rotated to transport the web 20 past the coatingdeposition station 24. Pressurized gas is supplied to the passages 44,44′ and the continuously open exit portions 46, 46′ conduct thispressurized gas into the region between the web and the drum, therebyimproving thermal contact between the web and the drum and cooling ofthe web. As the drum rotates, individual ones of the exit portions 46,46′ move repeatedly between the coating region and the sealed region.When the exit portions 46, 46′ are in the coating region, the web 20presses against the drum 14 adjacent the exit portions 46, 46′ to reducethe leakage of pressurized gas into the coating region 28. When the exitportions 46, 46′ are aligned with the sealed region 30, the seals 34,34′ minimize the undesired flow of pressurized gas out of the exitportions 46, 46′ into the coating region.

This invention can be adapted for use with the widest variety of drums,vacuum chambers, coating deposition stations, and sources of pressurizedgas. All of these elements can be modified widely as appropriate to fitthe intended application. For example, the drum can be provided with anydesired axial length, and the axial length can be smaller or larger thanthe diameter of the drum. The angle over which the web is in contactwith the drum can also be varied widely, including small angles of wrap(less than 90°) as well as large angles of wrap (greater than 180°).This invention is not limited to any particular material for the web.Steel, plastic or other materials can be used.

Also, the preferred embodiments described above work as intended whetherthe drum is rotated in the clockwise or the counter clockwise direction.Many vacuum coating devices today include the ability to coat in bothdirections, and the preferred embodiments described above are wellsuited for use in such devices.

By way of example only and without intending any limitation on thefollowing claims, the following preferred dimensions and materials havebeen found suitable in one example. The passages 44 can be provided withcross-sectional dimensions of 10 mm deep by 2 mm wide and acenter-to-center spacing along the axial length of the drum of 25 mm.The exit portions 46 can be circular in cross section with a diameter of0.3 mm and a length of 2 mm. The pressure P1 can be equal to 10 Torr,and the vacuum in the vacuum chamber 12 can be maintained at a pressureof 5×10−4 Torr. The drum 14 can be formed in the conventional manner ofcarbon steel with a hard, chrome-coated web support surface 22 polishedto a roughness average (Ra) of 0.2 micrometers.

As used herein, the term “set” is intended to indicate one or more. Theterm “substantially prevent” as applied to leakage is intended toindicate reduced leakage at an acceptably low level for the vacuumsystem being used. The term “source” as applied to pressurized gasincludes a source of a single gas at a single pressure as well as asource of one or more gasses at two or more pressures.

The embodiments described above will open new process avenues because anelevated, controlled gas pressure is provided behind the web in apractical manner. The foregoing detailed description has discussed onlya few of the many forms that this invention can take. For this reason,this detailed description is intended by way of illustration and notlimitation. It is only the following claims, including all equivalents,that are intended to define the scope of this invention.

1. A web coating apparatus comprising: a rotatable drum comprising a websupport surface and a plurality of passages, each passage comprising atleast one exit portion positioned in the web support surface, each exitportion being in continuous fluid communication with a region externalof and immediately adjacent to the web support surface; a source ofpressurized gas in fluid communication with the passages; a coatingdeposition station positioned adjacent the web support surface; and aset of seals positioned near the web support surface, said set of sealsforming a sealed region extending over an arc of the drum angularlyspaced from the coating deposition station as the drum rotates relativeto the set of seals, said set of seals operative to substantiallyprevent pressurized gas from escaping from the exit portions in thesealed region to a coating region adjacent the coating depositionstation; said exit portions cyclically moving through the sealed regionand the coating region as the drum rotates; wherein each seal sealsagainst the web support surface.
 2. The invention of claim 1 furthercomprising: first and second winding hubs rotatably mounted adjacent thedrum; a web wrapped around the hubs and wrapped partially around thedrum on the web support surface in the coating region, said web spacedfrom the web support surface over at least a major part of the sealedregion.
 3. The invention of claim 1 further comprising a manifold influid communication with the source of pressurized gas, wherein eachseal is coupled with the manifold to seal the manifold to the websupport surface, and wherein the manifold conducts pressurized gas fromthe source into the passages via the exit portions in the sealed region.4. The invention of claim 3 wherein each passage comprises an elongatedpassage extending circumferentially around the drum and in fluidcommunication with at least one of the exit portions.
 5. The inventionof claim 4 wherein the manifold and the elongated passages extend over aworking axial width of the drum such that pressurized gas at asubstantially uniform, elevated pressure is introduced between the weband the web support surface across the working axial width of the drum.6. The invention of claim 1 wherein each passage comprises an elongatedpassage extending circumferentially around the drum and in fluidcommunication with at least one of the exit portions.
 7. The inventionof claim 1 wherein each passage comprises an elongated portion extendingaxially along the drum and in fluid communication with at least one ofthe exit portions.
 8. The invention of claim 1 wherein the exit portionscomprise elongated slits.
 9. The invention of claim 1 wherein the exitportions comprise tubes.
 10. The invention of claim 1 wherein thedeposition station comprises multiple deposition sources.
 11. Theinvention of claim 1 wherein the source of pressurized gas comprises agas pressure regulation system.
 12. The invention of claim 1 furthercomprising a plurality of manifolds, each manifold in fluidcommunication with the source of pressurized gas at a respectivepressure, wherein each seal is coupled with the manifolds to seal themanifolds to the web support surface, and wherein each manifold conductspressurized gas at the respective pressure into respective ones of thepassages via the respective exit portions in the sealed region.
 13. Amethod for coating a web in a web coating process, said methodcomprising: (a) providing a coating deposition station and a rotatabledrum, said drum comprising a web support surface and a plurality ofpassages, each passage comprising at least one exit portion positionedin the web support surface, each exit portion being in continuous fluidcommunication with a region external of and immediately adjacent to theweb support surface; (b) transporting a web pressed against the websupport surface past the coating deposition station; (c) supplying apressurized gas via the passages and the exit portions into a regionbetween the web and the web support surface, thereby improving thermalcontact of the web with the drum; and (d) sealing the exit portions notcovered with the web with at least one seal that seals against the websupport surface as the drum rotates relative to the seal.